Make a curve be exactly a perfect slice of a circle
Try the following: 


Model the train Rail as linear, uncurved Shape.
Insert a Circle with add-mesh-Circle. Use the Number of verts as a multiplier of 360 degrees,  e.g. 36 verts.
Delete as many verts as you Need until you geht the Desired bow.
In Object Mode,  use Alt+c to convert the Circle bow Into a Curve. 
Apply a Curve modifier to the linear Rail from 1., Using the curve from 4. as Curve. 


Be aware of that both objects Need to have applied scale,  rotation and Location.

I Hope this Helps. 

Kind regards,  Karl Andreas Groß